Internal combustion engines can carry out combustion by repeatedly admitting air and fuel into the cylinder, combusting the air and fuel, and exhausting the products of combustion from the cylinder. Some engines may perform these processes by operating in what may be referred to as a two stroke cycle, which corresponds to a combustion event being performed every two strokes of the cylinder's piston. Other engines may perform these processes by instead operating in a four stroke cycle, which corresponds to a combustion event every four strokes of the piston.
The four stroke cycle may be advantageously used in some examples to provide increased engine efficiency as compared to a comparable two stroke cycle, since air and fuel may be more thoroughly mixed in the four stroke cycle. However, the two stroke cycle may be advantageously used in other examples to provide increased power or torque output as compared to the four stroke cycle, since combustion within the cylinder can be performed at twice the frequency as the four stroke cycle for a given engine speed.
To take advantage of both the two stroke and four stroke cycles, some engines can be configured to selectively transition between two stroke and four stroke operation. For example, U.S. Pat. No. 6,257,176 depicts an engine that can be operated in either a two stroke or four stroke cycle. However, the inventor has recognized some issues associated with this approach.
As one example, the inventor has recognized that some exhaust aftertreatment devices for treating an exhaust stream produced by the engine may operate most effectively when receiving a specific exhaust gas composition. For example, some exhaust aftertreatment devices may most efficiently treat exhaust gas that corresponds to a stoichiometric or near stoichiometric ratio of air and fuel. Yet, selectively operating a cylinder in a two or four stroke cycle under different conditions can present challenges for air and fuel control at the aftertreatment device. As one example, the amount of exhaust gases remaining in the cylinder and the amount of intake air passing through the cylinder without undergoing combustion may differ considerably in each of the two and four stroke cycles, thereby potentially causing substantial variations in exhaust gas composition where a similar control strategy is employed during two stroke and four stroke cycles.
To address some of these issues, the inventor herein has provided a method of operating an internal combustion engine. As one example, the method may include: during a first operating condition, operating a cylinder of the engine in a two stroke cycle to combust a first mixture of air and fuel, and adjusting an opening overlap between an intake poppet valve and an exhaust poppet valve of the cylinder to vary a composition of exhaust gases exhausted by the cylinder via the exhaust poppet valve; and during a second operating condition, operating a cylinder of the engine in a four stroke cycle to combust a second mixture of air and fuel, and adjusting a relative amount of fuel contained in the second mixture of air and fuel to vary the composition of exhaust gases exhausted by the cylinder via the exhaust poppet valve.
In some examples, the combustion chamber may be supplied with a richer air and fuel mixture during the two stroke cycle than during the four stroke cycle while obtaining a similar air-fuel ratio at the exhaust aftertreatment device by adjusting, via valve timing and/or engine boosting, an amount of un-combusted intake air (e.g., blow-by) that passes through the cylinder during the two stroke cycle. In this way, the resulting exhaust gas composition that is received at the exhaust aftertreatment device can be better controlled to a target value even as the engine is transitioned between two stroke and four stroke cycles, and even as the engine continues to operate in the two stroke and four stroke cycles.